1. Field of the Invention
The present invention relates to an electric stringed instrument, e.g., an electric guitar or an electric piano, and more particularly to an electric stringed instrument having an electromagnetic force producing unit, e.g., an electromagnetic driver for sustaining the vibration of a string or a loudspeaker for outputting an amplified sound converted from an electrical signal produced by the vibration of a string, and further substantially causing no high frequency noise produced by so-called "magnetic feedback".
2. Description of the Prior Art
Generally, an electric stringed instrument, e.g., an electric guitar or an electric piano, has plural strings and a transducer, i.e., a pickup for converting the vibration of a string to an electrical signal. There are two well-known types of pickups. One is an electromagnetic type of pickup and the other is a piezoelectric type of pickup. Particularly, in a case of an electric guitar, the electromagnetic type of pickup has been used since the introduction of electric guitars, because by using the electromagnetic type of pickup it is easy to obtain a sound characteristic that emphasizes a medium sound region, by mounting the electromagnetic type of pickup on a body of an electric guitar and processing the resultant electrical signal.
In recent years, an attempt to use an electromagnetic transducer for exciting a string has been made and has actually been put into practice. This transducer utilizes an inverse principle of operation of an electromagnetic pickup which converts the vibration of a string to an electrical signal. Namely, an electrical signal detected by a pickup is amplified by an amplifier, and the amplified signal is applied to an electromagnetic driver, and thereby the electromagnetic driver emits magnetic energy to a string. By the magnetic energy emitted from the electromagnetic driver the string is self-excited, and thereby the vibration of the string is sustained for a long time.
However, in order to excite a string through an electromagnetic driver, the electromagnetic driver, i.e., an electromagnetic force producing unit, must emit a strong magnetic force in the vicinity of the string. Therefore, there is a problem that the magnetic field generated by the electromagnetic driver causes a so-called magnetic feedback which is introduced by a leakage of flux fed back from the electromagnetic driver to an electromagnetic pickup. The magnetic feedback causes an induced electromotive force of no use in the electromagnetic pickup, and thereby causes noise, e.g., an oscillation in a frequency range from about 1,000 Hz to 20,000 Hz.
Several prior arts disclose devices for reducing the magnetic feedback described above and sustaining the vibration of a string in relation to an electric stringed instrument. For example, Japanese Patent Publication No. 52-151022 and Japanese Utility Model Publication No. 53-139836 (Both filed by Roland Ltd.) disclose an electric guitar in which the strings of the guitar are connected to an electrical driving circuit mounted within the guitar, and when detecting the vibration of the strings at a pickup on the guitar, a positive feedback current output from the circuit flows through the strings as a part of the circuit, and so the strings in which the positive feedback current is flowing vibrate in cooperation with a magnet attached to a surface of the guitar. This type of guitar does not use an electromagnetic driver that converts an electrical signal detected by a pickup into a magnetic driving force utilizing a variation of magnetic flux corresponding to the signal and drives the metal string by the driving force. Thus, this type of guitar has the advantage of not generating the magnetic feedback described above. However, the guitar requires an external power supply to provide power for the self-driving strings, and further it must have a strong magnet to drive the strings in which the positive feedback current flows. The system also becomes large since the strings must be connected to the circuit. Therefore, the type of electric guitar described above needs to be designed as an exclusive instrument, and consequentially it is not practical to manufacture it on a commercial basis.
Another disclosure is described in Japanese Utility Model Publication No. 55-152597 (YAMAHA Ltd.). Although there is no description of magnetic feedback described above, a pickup shown in the specification and the drawings uses a light-emitting element and a light-intercepting element, and thus, there is no need to consider the magnetic feedback described above. However, this type of electric guitar does not use an electromagnetic pickup as described above so that a tone generated from this guitar is different from the tone of an electric guitar having a commonly used electromagnetic pickup.
Furthermore, U.S. Pat. No. 4,941,388 (Hoover, et al.) discloses a constitution that has an electromagnetic pickup and an electromagnetic driver in order to sustain the vibration of strings of an electric guitar without using the arrangement described above. In this constitution, an unbalancing device for putting a magnetic balance between the electromagnetic pickup and the electromagnetic driver out of balance is used to reduce the magnetic feedback described above, and as a particularly effective method, an embodiment using a shunting plate is disclosed. An electric guitar having a device for reducing the magnetic feedback by using the shunting plate is put into practical use and a device by the name of "Sustaniac" is available on the market. However, even if the shunting plate is used, a part of the magnetic feedback that cannot be completely absorbed into the shunting plate remains. In order to reduce the magnetic flux from the electromagnetic driver to the electromagnetic pickup as much as possible, the design of the shunting plate is greatly limited to precisely match the shunting plate to magnetic characteristics and winding orientation of pole pieces of the electromagnetic pickup and the electromagnetic driver, and further in this type of guitar using the shunting plate, only a humbucking type of pickup can be utilized.
Furthermore, in recent years an electric stringed instrument having an amplifier and a loudspeaker in a body thereof has been proposed. In the electric stringed instrument described above, the vibration of a string is converted to an electric signal by an electromagnetic pickup, and the electric signal is amplified by an amplifier and then the amplified signal is output as a guitar sound from a loudspeaker. Since this type of electric stringed instrument that has an amplifier and a loudspeaker in its body can output enough sound level without using an external amplifier, it is very portable in that it can be played anywhere. Therefore, the electric stringed instrument described above is very convenient for playing music or enjoying the sound of the instrument easily. However, in such an application of the electric stringed instrument to an electric guitar, a distance between a loudspeaker and an electromagnetic pickup in a guitar body must be short because of a limited absolute size of the guitar body, and magnetic flux emitted from a voice coil of the loudspeaker in use easily penetrates the electromagnetic pickup as well as a leakage flux emitted from an electromagnetic driver described above, and thereby the magnetic feedback described above is caused so that a sound output from the loudspeaker involves a high frequency noise as described above.